Most oil tankers, on completion of cargo discharge, take ballast water directly into the almost empty tanks from which the cargo has recently been discharged. This is in order to stabilize the ship for its return journey to the loading terminal. The amount of ballast loaded is approximately one-third of the cargo carrying capacity of the tanker. The ballast also helps to immerse the hull, propeller, and rudder, in the sea, thereby improving the manoeuvring characteristics of the ship in the light (unloaded) condition. An unfortunate side effect of the above system is that the ballast water mixes with any oil residues remaining in the tanks from the cargo. In most cases the amount of residue remaining is relatively large, and this eventually has to be pumped into the sea together with the ballast water when the latter is discharged prior to loading another cargo.
In order to eliminate the above source of pollution, proposed international maritime rules will require most oil tankers to be fitted with a segregated ballast system, i.e. whereby the oil cargo and its residues are kept physically separated from the ballast water at all times. An obvious method to achieve this separation, is to have separate tanks and pumping systems for the cargo oil and the ballast. However, this means that in a tanker that would historically have carried (say) 300,000 tons of cargo, it will now only be able to carry approximately 200,000 tons, as approximately 100,000 tons of tank space will have to be dedicated to the carriage of ballast water.
The result of the foregoing will be a direct and obvious loss of cargo generated revenue to the shipowner or ship charterer.
It is with a view to overcoming the above problem, while still retaining a 100% pollution free operation, that the system according to the present invention has been devised.